In a heavy duty, industrial-type propeller fan construction it has heretofore been customary for the impeller thereof to embody a solid steel center piece to which a plurality of symmetrically arranged blades of sheet steel are affixed. The blades in such a construction are twisted so as to impart a desired pitch thereto relative to the plane of rotation. Such an arrangement, however, is in contrast to the conventional, light duty fan construction wherein the center piece, or spider member, of the impeller is provided with a plurality of wings or extensions twisted so as to impart a pitch to the flat blades which are affixed thereto. It has been found in the latter type of construction that the operating efficiency thereof is in the order of 40%-50%. In the heavy duty propeller fan construction, as aforedescribed, the operating efficiency thereof is also very poor. The poor efficiency in these prior constructions are deemed attributable to one or more of the following factors: (a) the various components of the impeller have an inordinate amount of weight thereby requiring greater horsepower input to effect rotation of the impeller, (b) power loss results from an excessive turbulent air condition being created in the vicinity of the spider member, particularly where the latter is a solid steel plate, (c) an undesirable air stall condition is produced in the vicinity of the blade outer ends due to the amount of clearance of the outer ends with respect to the opening or housing in which the propeller fan is placed, and (d) proper balancing of the fan impeller was difficult to attain.
With regard to balancing of the fan impeller, particularly in the larger size units, it was customary to apply predetermined weights on the blades by welding or riveting, which involved special equipment and was a costly and time-consuming operation.